So, about 4K60 on the Pixel 4: the limited storage may not be the only reason why it isn't supported. Thread:
If you add 4K60 as a supported media profile in /vendor/etc/ media profiles.xml, you can unlock the 60fps option in GCam. Record a video and you'll quickly notice that the FPS fails to exceed 38. (1 tested this and the results weren't pretty.)
GCam modder savitar/defcomg said the Pixel 4 connects to both cameras at once in all modes, likely to allow for seamless switching betwen lenses when zooming. Because of this, the ISP in the 855 can't handle recording 4K60 on the main and 1080p60 on the secondary.
if you calculate the bandwidth used for 4K60 and use that value to account for 4K38, the remainder matches the IMX481's binned resolution at almost 60fps. 3840216060 497,664,000 3840/2160/38 182,476,800 / 2328/1310 59.38fps
In theory, if you found a way to kill the tele lens while using GCam, you should be able to record 4K60.
Again, thanks to savitar/defcomg and
@Cstark 27 for the research/analysis.